exhaust - correct

inlet manifold / throttlebody depends what ecu youre running. i dont think an ecu for a plastic manifold car which uses an air bleed type stepper motor will run the mechanical stepper motor the alloy manifold uses, and vice verca.

i think the engine looks themselves are interchangeable though, and can be completely seperated from the main car loom if you unclip a wire from the underbonnet fuse box.

not sure how different the r100 wiring is so dont want to give any advice that might be completely wrong because ive only done metros before.

your 1.4 flywheel should be fine. as its a late mpi it should allready have 4 crankshaft sensor pickups.

a 216 clutch will bolt to a 1.4 flywheel, though you may need to use bolts that arent 'shouldered' for the cover plate.

i have an exhaust manifold from a late mpi gti which you can have for free, unfortunatly the 2 pipes that go under the sump are very squashed, but might be repairable?